1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gas from internal combustion engines and the like, and more particularly to a catalyst suitable for eliminating nitrogen oxides exhausted from internal combustion engines and the like.
2. Description of the Prior Art
Nitrogen oxides (NO.sub.x) exhausted from internal combustion engines and the like cause photochemical smog or acid rain and the removal of nitrogen oxides from the sources is an urgent problem to be solved.
Conventionally, for exhaust gas from large-scale stationary sources like thermal power plants and the like, a selective catalytic reduction method is used which adds ammonia to the exhaust gas and uses a TiO.sub.2 -V.sub.2 O.sub.5 catalyst. For exhaust gas from gasoline engines of automobiles and the like, the three-way catalyst (TWC) method is applied in which the air/fuel ratio is controlled near to the stoichiometric quantity (A/F=14.6) and a Pt-Rh/Al.sub.2 O.sub.3 catalyst is used to eliminate NO.sub.x, carbon monoxide (CO), and hydrocarbons (HC) simultaneously.
On the other hand, recently, to prevent global warming, it has become necessary to control the emission of carbon dioxide (CO.sub.2) and it is demanded to put lean burn gasoline engines to practical use, but the three-way catalyst is not effective in treating the exhaust gas from lean burn gasoline engines.
Although diesel engines are essentially of the lean burn system, the elimination of floated particulate substances and NO.sub.x present in the exhaust gas from diesel engines is now an urgent problem to be solved.
Hereinafter, the engines of the lean burn system like these lean burn gasoline engines and diesel engines are generally called lean burn engines.
The exhaust gas from lean burn engines contains oxidizing agents such as oxygen (O.sub.2) and NO.sub.x in excess of the stoichiometric quantity required for oxidation of reducing components such as HC, CO, and hydrogen (H.sub.2), and particularly contains oxygen in such a large quantity that the air/fuel ratio becomes 17 or over. It has long been a problem to be solved that NO.sub.x in exhaust gas is selectively eliminated in the presence of this excess oxygen without adding a special reducing agent such as ammonia.
Once, the use of iridium (Ir) catalysts supported on a porous inorganic oxide such as alumina (Al.sub.2 O.sub.3) for ridding exhaust gas having excess oxygen of NO.sub.x was suggested (Japanese Patent Publication (kokoku) Nos. 56-54173 (1981) and 57-13328 (1982) and U.S. Pat. No. 4,039,622 (1977)) . However, in Examples in the above-mentioned Japanese Patent Publications and U.S. Patent, the performance of catalysts is shown only under conditions wherein the concentration of oxygen in exhaust gas is 3% or less, i.e., the A/F ratio is less than 17, which means the suggestions are unsatisfactory, in terms of selectivity and life, to the elimination of NO.sub.x from exhaust gas containing excess oxygen equivalent to the air/fuel ratio of 17 or over from lean burn engines.
For the elimination of NO.sub.x from exhaust gas from lean burn engines, in recent years, for example, aluminosilicates on which a transition metal such as copper (Cu) is supported by ion exchange (U.S. Pat. No. 4,297,328 and Japanese Pre-examination Patent Publication (kokai) No. 63-100919 (1988)), metallosilicates (Japanese Pre-examination Patent Publication (kokai) Nos. 3-127628 (1991) and 3-229620 (1991), or silicoaluminophosphate catalysts (japanese Pre-examination Patent Publication (kokai) No. 1-112488 (1989) (hereinafter referred generally to as metallosilicate catalysts) are reported to have certain degrees of selectivity. However, these catalysts are attended with such fatal defects that the effective temperature range in which NO.sub.x is eliminated is narrowly restricted and that the catalyst is deteriorated irreversibly within few hours under a high temperature of about 650.degree. to 700.degree. C. due to the steam present in exhaust gas and the activity is extremely lowered.